Fiber optic intracore Bragg gratings have been used in prior sensing systems as reliable, localized sensors of strain, temperature and other physical parameters. The wavelength encoding of strain and temperature in these Bragg grating sensors make them suited for monitoring structures to track strain and temperature over periods of time. Such sensors are not affected by electrical interference as compared to electrical based sensors. However, the prior systems which have employed such sensors have been very expensive and inflexible in their ability to detect different parameters. In addition, such prior systems were high in volume and weight (e.g., large and heavy) which made them unacceptable in mobile applications.
An example of such a prior art two channel system employing Bragg grating sensors is illustrated in FIG. 1. Each channel includes a sensor array SA1, SA2 which is embedded in or attached to the structure to be monitored, such as within a concrete deck support girder for a bridge. Each sensor of each array reflects light of a particular bandwidth which is different from the bandwidth of the other sensors in the array. For example, a first sensor of array SA1 may be detecting strain and reflecting light within a bandwidth having a wavelength of 1295-1297 nanometers (nm) whereas a second sensor of sensor array SA2 may be detecting temperature and reflecting light within a bandwidth having a wavelength of 1329-1331 nanometers (nm), the specific wavelength of reflection being related to the strain or temperature of the sensor with changes tracking generally linearly. Consequently, the sensors of each array of each channel must be illuminated by light from a different, separate broad band light source BBLS1 which emits light corresponding to the sensor's reflective operating bandwidth.
Optical fibers transmit the light from the source via a 2.times.1 coupler to the sensor array and transmit the reflected light from the sensors of the array via the coupler to a tunable filter TF1, TF2 driven by a waveform generator WG which is scanned to detect a narrow band of reflected light. A peak detector PD1, PD2 detects the peak of any reflected light within the narrow band of the filter for each channel. Each peak detector generates a digital pulse representative of the peak of the reflected light. The digital pulses generated during scanning are used as a trigger for the A/D converter and thus are converted to a value which is proportional to a particular wavelength. Knowing the tunable filter's response, the converted values can be identified as an exact wavelength. Using a model of the sensor's relationship of wavelength to a particular parameter, a value based on this parameter can be made.
Such systems have been found to be very expensive, primarily because of the cost associated with the various light sources needed to illuminate each sensor array with light of the appropriate bandwidth and multiple tunable filters to locate the sensors. In addition, the need for various or multiple light sources tends to add the significant weight and volume of such prior art systems which limit the applicability of the system for use in mobile applications or other environments in which a light weight or compact monitoring system is needed or desired.
There is a need for an improved optical fiber sensor system which avoids these prior art deficiencies and would be useful in mobile systems such as systems which monitor the structural integrity of aircraft and systems by monitoring parameters indicating safe conditions such as structural monitors for bridges. In addition, there is a need for an improved optical fiber sensor system which could be used in telecommunications.